. >> how often do you use online reviews? >> every day, all the time. >> do you trust those online reviews? >> um... um... >> reporter: it seems everybody uses them, but they don't necessarily trust them. >> would you be surprised to hear that four out of ten online reviews are phony? >> oh, i believe it. >> reporter: well, in an effort to change that, yelp is going undercover. >> we saw ads on places like craigslist and others, advertising, please write reviews and i'll pay you. >> reporter: so in a sort of sting operation, yelp employees responded to the ads, which offer consumers as much as $200 in exchange for great reviews. but instead of one of those, they got one of these. a consumer alert level that will remain on their page for the next three months with a link to evidence that the business tried to buy bogus reviews. >> we found all kinds of businesses trying to do this, moving companies, jeweler, tour company operator. >> reporter: and, yes, at least one of the companies was a paid advertiser which not only got sla